compiler/libec: Improved detection of incompatible pointer types

compiler/libec: Improved detection of incompatible pointer types
- Added an additional check based on type size, as the compiler
  would previously not warn for assigning a uint16 * to a uint *
- Fixed type size computation for C structs (as included in C headers)
- Fixed FindType()'s null check to be for the context passed in rather than 'curContext'
  (assuming that was the original intent)